Blender

This section of the blog will discuss issues related to Blender, the open source 3D environment that is making much progress and which you can start learning for absolutely free.

a pocket watch modeled with Blender

Start with blender.org and get the latest version. In July 2015 the latest version is 2.74, and you can download a Windows (32/64 bit), Linux, and MacOS version. No matter which you get, your work can be edited in any other version, and the user interface will be (almost) the exact same thing in all three.

You also should get an account at Blender Artists because they have extensive tutorial lists and a forum where every Blender question will be answered.